Some types of these electric power steering systems are designed to generate the assist torque by a motor and control the assist torque using: torque created based on the driver's turning of a steering wheel; and self aligning torque. The torque created based on the driver's turning of the steering wheel is measured by a torque sensor. The torque sensor is normally disposed between an input shaft rotatable together with the steering wheel and an output shaft, such as a pinion shaft. The torque sensor includes a torsion bar coupling the input shaft and the output shaft, and measures the torque, referred to as “steering torque”, based on a torsion angle of the torsion bar by the driver's turning of the steering wheel.
The self aligning torque is the torque (force) that causes a tire/wheel to tend to rotate around its vertical axis. For example, when there is a slip angle of a tire/wheel, the self aligning torque created by the tire/wheel causes the tire/wheel to tend to rotate around its vertical axis toward its straight ahead position.
For example, US Patent Application Publication No. US 2005/0103561 corresponding to Japanese Patent Application Publication No. 2003-200844 discloses an example of these types of electric power steering systems.
The electric power steering system disclosed in the US Patent Application Publication is designed to estimate, by a disturbance observer, the self aligning torque ES using a commanded current Ir for a motor for generating the assist torque and an angular velocity ω of the motor as input parameters. The electric power steering system is also designed to estimate steering reaction force AT using the sum (SatE) of the output Ttb of a transfer function of the spring coefficient KTb of the torsion bar and the self aligning torque ES as an input parameter.
Then, the electric power steering system is designed to feedback the steering reaction force AT through a first feedback loop and the self aligning torque ES through a second feedback loop to the calculation of the assist torque. Specifically, the electric power system is designed to calculate the deviation (AT-T) of the steering torque T from the steering reaction force AT, and input, to a control unit, the deviation (AT-T) so that the control unit controls the motor to thereby generate the assist torque that reduces the deviation (AT-T).